1. Field of the Invention
This invention relates generally to methods and apparatus for desorption and ionization of analytes for the purpose of subsequent scientific analysis by such methods, for example, as mass spectrometry or biosensors. More specifically, this invention relates to the field of mass spectrometry, especially to the type of matrix-assisted laser desorption/ionization, time-of-flight mass spectrometry used to analyze macromolecules, such as proteins or biomolecules.
2. Description of the Prior Art
Generally, analysis by mass spectrometry involves the vaporization and ionization of a small sample of material using a high energy source, such as a laser, including a laser beam. The material is vaporized from the surface of a probe tip by the laser beam, and in the process, some of the individual molecules are ionized by the gain of a proton. The positively charged ionized molecules are then accelerated through a short high voltage field and let fly into a high vacuum chamber, at the far end of which they strike a sensitive detector surface. Since the time of flight is a function of the mass of the ionized molecule, the elapsed time between ionization and impact can be used to determine the molecule's mass which, in turn, can be used to identify the presence or absence of known molecules of specific mass.
All known prior art procedures which present proteins or other large biomolecules on a probe tip for laser desorption/ionization time-of-flight mass spectrometry rely on a crystalline solid mixture of the protein or other analyte molecule in a large excess of acidic matrix material deposited on the bare surface of a metallic probe tip. (The sample probe tip typically is metallic, either stainless steel nickel plated material or platinum). Immobilizing the analyte in such a matrix was thought to be necessary in order to prevent the destruction of analyte molecules by the laser beam. The laser beam strikes the mixture on the probe tip and its energy is used to vaporize a small portion of the matrix material along with some of the embedded analyte molecules. Without the matrix, the analyte molecules are easily fragmented by the laser energy, so that the mass, and identity, of the original macromolecule is very difficult to determine.
This prior art procedure has several limitations which have prevented its adaptation to automated protein or other macrobiological molecules analysis. First, in a very crude sample it is necessary to partially fractionate (or otherwise purify the sample as much as possible) to eliminate the presence of excessive extraneous materials in the matrix/analyte crystalline mixture. The presence of large quantities of components may depress the signal of the targeted analyte. Such purification is time-consuming and expensive and would be very difficult to do in an automated analyzer.
Second, while the amount of analyte material needed for analysis by the prior art method is not large (typically in a picomole range), in some circumstances, such as tests on pediatric patients, analyte fluids are available only in extremely small volumes (microliters) and may be needed for performing several different analyses. Therefore, even the small amount needed for preparation of the analyte/matrix crystalline mixture for a single analysis may be significant. Also, only a tiny fraction (a few thousandths or less) of analyte used in preparing the analyte/matrix mixture for use on the probe tip is actually consumed in the mass spectrometry analysis. Any improvement in the prior art procedure which made it possible to use much less analyte to conduct the test would be highly advantageous in many clinical areas.
Third, the analyte protein, or other macromolecule, used in preparing the analyte matrix for use on the probe tip is not suitable for any subsequent chemical tests or procedures because it is bound up in the matrix material. Also, all of the matrix material used to date is strongly acidic, so that it would affect many chemical reactions which might be attempted on the mixture in order to modify the analyte molecules for subsequent examination. Any improvement in the procedure which made it possible to conduct subsequent chemical modifications or reactions on the analyte molecules, without removing them from the matrix or the probe tip, would be of enormous benefit to researchers and clinicians.
Additional limitations in the prior art included problems with matrix use such as:
(1) formation of analyte-matrix complex (referred to as “matrix adduct” which interferes with the accuracy of analyte measurement;
(2) inability to wash away contaminants present in analyte or matrix (e.g., other proteins or salts);
(3) formation of analyte-salt ion adducts;
(4) less than optimum solubility of analyte in matrix;
(5) signal (molecular ion) suppression “poisoning” due to simultaneous presence of multiple components; and
(6) selective analyte desorption/ionization.
There are a number of problems and limitations with the prior art methods. Prior investigators, including Karas and Hillenkamp have reported a variety of techniques for analyte detection using mass spectroscopy, but these techniques suffered because of inherent limitations in sensitivity and selectivity of the techniques, specifically including limitations in detection of analytes in low volume, undifferentiated samples. The “Hillenkamp-Karas” articles that pertain to this field of invention are:
1. Hillenkamp, “Laser Desorption Mass Spectrometry: Mechanisms, Techniques and Applications”; Bordeaux Mass Spectrometry Conference Report, 1988, pages 354–362.
2. Karas and Hillenkamp, “Ultraviolet Laser Desorption of Proteins Up to 120,000 Daltons”, Bordeaux Mass Spectrometry Conference Report, 1988, pages 416, 417.
3. Karas and Hillenkamp, “Laser Desorption Ionization of Proteins With Molecular Masses Exceeding 10,000 Daltons”, Analytical Chemistry, 60. 2299,2301 July 1988.
4. Karas, Ingendoh, Bahr and Hillenkamp, “UV-Laser Desorption/Ionization Mass Spectrometry of Femtomol Amounts of Large Proteins”, Biomed. Environ. Mass Spectrum 18:841–843 (1989).
The use of laser beams in time-of-flight mass spectrometers is shown, for example, in U.S. Pat. Nos. 4,694,167; 4,686,366, 4,295,046, and 5,045,694, incorporated by reference.
The first successful molecular mass measurements of intact peptides and small proteins (only up to about 15 kDa) by any form of mass spectrometry were made by bombarding surfaces with high energy particles (plasma desorption and fast atom bombardment mass spectrometry); this breakthrough came in 1981 and 1982. Improvements came in 1985 and 1986, however, yield (signal intensities), sensitivity, precision, and mass accuracy remained relatively low. Higher molecular mass proteins (about 20 to 25 kDa) were not observed except on rare occasions; proteins representing average molecular weights (approximately 70 kDa) were not ever observed with these methods. Thus, evaluation of most proteins by mass spectrometry remains unrealized.
In 1988, Hillenkamp and his coworkers used UV laser desorption time-of-flight mass spectrometry and discovered that when proteins of relatively high molecular mass were deposited on the probe tip in the presence of a very large molar excess of an acidic, UV absorbing chemical matrix (nicotinic acid) they could be desorbed in the intact state. This new technique is called matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry. Note that laser desorption time-of-flight mass spectrometry (without the chemical matrix) had been around for some time, however, there was little or no success determining the molecular weights of large intact biopolymers such as proteins and nucleic acids because they were fragmented (destroyed) upon desorption. Thus, prior to the introduction of a chemical matrix, laser desorption mass spectrometry was essentially useless for the detection of specific changes in the mass of intact macromolecules (see below). Note that the random formation of matrix crystals and the random inclusion of analyte molecules in the solid solution is prior art.